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Concept
The area of physics known as acoustics is devoted to the study of the production, transmission, and reception of sound. Thus, wherever sound is produced and transmitted, it will have an effect somewhere, even if there is no one present to hear it. The medium of sound transmission is an all-important, key factor. Among the areas addressed within the realm of acoustics are the production of sounds by the human voice and various instruments, as well as the reception of sound waves by the human ear.
How It Works
Wave Motion and Sound Waves
Sound waves are an example of a larger phenomenon known as wave motion, and wave motion is, in turn, a subset of harmonic motion—that is, repeated movement of a particle about a position of equilibrium, or balance. In the case of sound, the "particle" is not an item of matter, but of energy, and wave motion is a type of harmonic movement that carries energy from one place to another without actually moving any matter.
Particles in waves experience oscillation, harmonic motion in one or more dimensions. Oscillation itself involves little movement, though some particles do move short distances as they interact with other particles. Primarily, however, it involves only movement in place. The waves themselves, on the other hand, move across space, ending up in a position different from the one in which they started.
A transverse wave forms a regular up-and-down pattern in which the oscillation is perpendicular to the direction the wave is moving. This is a fairly easy type of wave to visualize: imagine a curve moving up and down along a straight line. Sound waves, on the other hand, are longitudinal waves, in which oscillation occurs in the same direction as the wave itself.
These oscillations are really just fluctuations in pressure. As a sound wave moves through a medium such as air, these changes in pressure cause the medium to experience alternations of density and rarefaction (a decrease in density). This, in turn, produces vibrations in the human ear or in any other object that receives the sound waves.
Properties of Sound Waves
Cycle and Period
The term cycle has a definition that varies slightly, depending on whether the type of motion being discussed is oscillation, the movement of transverse waves, or the motion of a longitudinal sound wave. In the latter case, a cycle is defined as a single complete vibration.
A period (represented by the symbol T) is the amount of time required to complete one full cycle. The period of a sound wave can be mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength.
The Speed of Sound in Various Media
People often refer to the "speed of sound" as though this were a fixed value like the speed of light, but, in fact, the speed of sound is a function of the medium through which it travels. What people ordinarily mean by the "speed of sound" is the speed of sound through air at a specific temperature. For sound traveling at sea level, the speed at 32°F (0°C) is 740 MPH (331 m/s), and at 68°F (20°C), it is 767 MPH (343 m/s).
In the essay on aerodynamics, the speed of sound for aircraft was given at 660 MPH (451 m/s). This is much less than the figures given above for the speed of sound through air at sea level, because obviously, aircraft are not flying at sea level, but well above it, and the air through which they pass is well below freezing temperature.
The speed of sound through a gas is proportional to the square root of the pressure divided by the density. According to Gay-Lussac's law, pressure is directly related to temperature, meaning that the lower the pressure, the lower the temperature—and vice versa. At high altitudes, the temperature is low, and, therefore, so is the pressure; and, due to the relatively small gravitational pull that Earth exerts on the air at that height, the density is also low. Hence, the speed of sound is also low.
It follows that the higher the pressure of the material, and the greater the density, the faster sound travels through it: thus sound travels faster through a liquid than through a gas. This might seem a bit surprising: at first glance, it would seem that sound travels fastest through air, but only because we are just more accustomed to hearing sounds that travel through that medium. The speed of sound in water varies from about 3,244 MPH (1,450 m/s) to about 3,355 MPH (1500 m/s). Sound travels even faster through a solid—typically about 11,185 MPH (5,000 m/s)—than it does through a liquid.
Frequency
Frequency (abbreviated f) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894) and a Hertz is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.)
The human ear is capable of hearing sounds from 20 to approximately 20,000 Hz—a relatively small range for a mammal, considering that bats, whales, and dolphins can hear sounds at a frequency up to 150 kHz. Human speech is in the range of about 1 kHz, and the 88 keys on a piano vary in frequency from 27 Hz to 4,186 Hz. Each note has its own frequency, with middle C (the "white key" in the very middle of a piano keyboard) at 264 Hz. The quality of harmony or dissonance when two notes are played together is a function of the relationship between the frequencies of the two.
Frequencies below the range of human audibility are called infrasound, and those above it are referred to as ultrasound. There are a number of practical applications for ultrasonic technology in medicine, navigation, and other fields.
Wavelength
Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength, and vice versa. Thus, a frequency of 20 Hz, at the bottom end of human audibility, has a very large wavelength: 56 ft (17 m). The top end frequency of 20,000 Hz is only 0.67 inches (17 mm).
There is a special type of high-frequency sound wave beyond ultrasound: hypersound, which has frequencies above 107 MHz, or 10 trillion Hz. It is almost impossible for hypersound waves to travel through all but the densest media, because their wavelengths are so short. In order to be transmitted properly, hypersound requires an extremely tight molecular structure; otherwise, the wave would get lost between molecules.
Wavelengths of visible light, part of the electromagnetic spectrum, have a frequency much higher even than hypersound waves: about 109 MHz, 100 times greater than for hypersound. This, in turn, means that these wavelengths are incredibly small, and this is why light waves can easily be blocked out by using one's hand or a curtain.
The same does not hold for sound waves, because the wavelengths of sounds in the range of human audibility are comparable to the size of ordinary objects. To block out a sound wave, one needs something of much greater dimensions—width, height, and depth—than a mere cloth curtain. A thick concrete wall, for instance, may be enough to block out the waves. Better still would be the use of materials that absorb sound, such as cork, or even the use of machines that produce sound waves which destructively interfere with the offending sound.
Amplitude and Intensity
Amplitude is critical to the understanding of sound, though it is mathematically independent from the parameters so far discussed. Defined as the maximum displacement of a vibrating material, amplitude is the "size" of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity, commonly known as "volume," which is the rate at which a wave moves energy per unit of a cross-sectional area.
Intensity can be measured in watts per square meter, or W/m2. A sound wave of minimum intensity for human audibility would have a value of 10−12, or 0.000000000001, W/m2. As a basis of comparison, a person speaking in an ordinary tone of voice generates about 10−4, or 0.0001, watts. On the other hand, a sound with an intensity of 1 W/m2 would be powerful enough to damage a person's ears.
Real-Life Applications
Decibel Levels
For measuring the intensity of a sound as experienced by the human ear, we use a unit other than the watt per square meter, because ears do not respond to sounds in a linear, or straight-line, progression. If the intensity of a sound is doubled, a person perceives a greater intensity, but nothing approaching twice that of the original sound. Instead, a different system—known in mathematics as a logarithmic scale—is applied.
In measuring the effect of sound intensity on the human ear, a unit called the decibel (abbreviated dB) is used. A sound of minimal audibility (10−12 W/m2) is assigned the value of 0 dB, and 10 dB is 10 times as great—10−11 W/m2. But 20 dB is not 20 times as intense as 0 dB; it is 100 times as intense, or 10−10 W/m2. Every increase of 10 dB thus indicates a tenfold increase in intensity. Therefore, 120 dB, the maximum decibel level that a human ear can endure without experiencing damage, is not 120 times as great as the minimal level for audibility, but 1012 (1 trillion) times as great—equal to 1 W/m2, referred to above as the highest safe intensity level.
Of course, sounds can be much louder than 120 dB: a rock band, for instance, can generate sounds of 125 dB, which is 5 times the maximum safe decibel level. A gunshot, firecracker, or a jet—if one is exposed to these sounds at a sufficiently close proximity—can be as high as 140 dB, or 20 times the maximum safe level. Nor is 120 dB safe for prolonged periods: hearing experts indicate that regular and repeated exposure to even 85 dB (5 less than a lawn mower) can cause permanent damage to one's hearing.
Production of Sound Waves
Musical Instruments
Sound waves are vibrations; thus, in order to produce sound, vibrations must be produced. For a stringed instrument, such as a guitar, harp, or piano, the strings must be set into vibration, either by the musician's fingers or the mechanism that connects piano keys to the strings inside the case of the piano.
In other woodwind instruments and horns, the musician causes vibrations by blowing into the mouthpiece. The exact process by which the vibrations emerge as sound differs between woodwind instruments, such as a clarinet or saxophone on the one hand, and brass instruments, such as a trumpet or trombone on the other. Then there is a drum or other percussion instrument, which produces vibrations, if not musical notes.
Electronic Amplification
Sound is a form of energy: thus, when an automobile or other machine produces sound incidental to its operation, this actually represents energy that is lost. Energy itself is conserved, but not all of the energy put into the machine can ever be realized as useful energy; thus, the automobile loses some energy in the form of sound and heat.
The fact that sound is energy, however, also means that it can be converted to other forms of energy, and this is precisely what a microphone does: it receives sound waves and converts them to electrical energy. These electrical signals are transmitted to an amplifier, and next to a loudspeaker, which turns electrical energy back into sound energy—only now, the intensity of the sound is much greater.
Inside a loudspeaker is a diaphragm, a thin, flexible disk that vibrates with the intensity of the sound it produces. When it pushes outward, the diaphragm forces nearby air molecules closer together, creating a high-pressure region around the loudspeaker. (Remember, as stated earlier, that sound is a matter of fluctuations in pressure.) The diaphragm is then pushed backward in response, freeing up an area of space for the air molecules. These, then, rush toward the diaphragm, creating a low-pressure region behind the high-pressure one. The loudspeaker thus sends out alternating waves of high and low pressure, vibrations on the same frequency of the original sound.
The Human Voice
As impressive as the electronic means of sound production are (and of course the description just given is highly simplified), this technology pales in comparison to the greatest of all sound-producing mechanisms: the human voice. Speech itself is a highly complex physical process, much too involved to be discussed in any depth here. For our present purpose, it is important only to recognize that speech is essentially a matter of producing vibrations on the vocal cords, and then transmitting those vibrations.
Before a person speaks, the brain sends signals to the vocal cords, causing them to tighten. As speech begins, air is forced across the vocal cords, and this produces vibrations. The action of the vocal cords in producing these vibrations is, like everything about the miracle of speech, exceedingly involved: at any given moment as a person is talking, parts of the vocal cords are opened, and parts are closed.
The sound of a person's voice is affected by a number of factors: the size and shape of the sinuses and other cavities in the head, the shape of the mouth, and the placement of the teeth and tongue. These factors influence the production of specific frequencies of sound, and result in differing vocal qualities. Again, the mechanisms of speech are highly complicated, involving action of the diaphragm (a partition of muscle and tissue between the chest and abdominal cavities), larynx, pharynx, glottis, hard and soft palates, and so on. But, it all begins with the production of vibrations.
Propagation: Does It Make a Sound?
As stated in the introduction, acoustics is concerned with the production, transmission (sometimes called propagation), and reception of sound. Transmission has already been examined in terms of the speed at which sound travels through various media. One aspect of sound transmission needs to be reiterated, however: for sound to be propagated, there must be a medium.
There is an age-old "philosophical" question that goes something like this: If a tree falls in the woods and there is no one to hear it, does it make a sound? In fact, the question is not a matter of philosophy at all, but of physics, and the answer is, of course, "yes." As the tree falls, it releases energy in a number of forms, and part of this energy is manifested as sound waves.
Consider, on the other hand, this rephrased version of the question: "If a tree falls in a vacuum—an area completely devoid of matter, including air—does it make a sound?" The answer is now a qualified "no": certainly, there is a release of energy, as before, but the sound waves cannot be transmitted. Without air or any other matter to carry the waves, there is literally no sound.
Hence, there is a great deal of truth to the tagline associated with the 1979 science-fiction film Alien : "In space, no one can hear you scream." Inside an astronaut's suit, there is pressure and an oxygen supply; without either, the astronaut would perish quickly. The pressure and air inside the suit also allow the astronaut to hear sounds within the suit, including communications via microphone from other astronauts. But, if there were an explosion in the vacuum of deep space outside the spacecraft, no one inside would be able to hear it.
Reception of Sound
Recording
Earlier the structure of electronic amplification was described in very simple terms. Some of the same processes—specifically, the conversion of sound to electrical energy—are used in the recording of sound. In sound recording, when a sound wave is emitted, it causes vibrations in a diaphragm attached to an electrical condenser. This causes variations in the electrical current passed on by the condenser.
These electrical pulses are processed and ultimately passed on to an electromagnetic "recording head." The magnetic field of the recording head extends over the section of tape being recorded: what began as loud sounds now produce strong magnetic fields, and soft sounds produce weak fields. Yet, just as electronic means of sound production and transmission are still not as impressive as the mechanisms of the human voice, so electronic sound reception and recording technology is a less magnificent device than the human ear.
How the Ear Hears
As almost everyone has noticed, a change in altitude (and, hence, of atmospheric pressure) leads to a strange "popping" sensation in the ears. Usually, this condition can be overcome by swallowing, or even better, by yawning. This opens the Eustachian tube, a passageway that maintains atmospheric pressure in the ear. Useful as it is, the Eustachian tube is just one of the human ear's many parts.
The "funny" shape of the ear helps it to capture and amplify sound waves, which passthrough the ear canal and cause the eardrum tovibrate. Though humans can hear sounds over amuch wider range, the optimal range of audibility is from 3,000 to 4,000 Hz. This is because thestructure of the ear canal is such that sounds in this frequency produce magnified pressure fluctuations. Thanks to this, as well as other specific properties, the ear acts as an amplifier of sounds.
Beyond the eardrum is the middle ear, an intricate sound-reception device containing some of the smallest bones in the human body—bones commonly known, because of their shapes, as the hammer, anvil, and stirrup. Vibrations pass from the hammer to the anvil to the stirrup, through the membrane that covers the oval window, and into the inner ear.
Filled with liquid, the inner ear contains the semicircular canals responsible for providing a sense of balance or orientation: without these, a person literally "would not know which way is up." Also, in the inner ear is the cochlea, an organ shaped like a snail. Waves of pressure from the fluids of the inner ear are passed through the cochlea to the auditory nerve, which then transmits these signals to the brain.
The basilar membrane of the cochlea is a particularly wondrous instrument, responsible in large part for the ability to discriminate between sounds of different frequencies and intensities. The surface of the membrane is covered with thousands of fibers, which are highly sensitive to disturbances, and it transmits information concerning these disturbances to the auditory nerve. The brain, in turn, forms a relation between the position of the nerve ending and the frequency of the sound. It also equates the degree of disturbance in the basilar membrane with the intensity of the sound: the greater the disturbance, the louder the sound.
Where to Learn More
Adams, Richard C. and Peter H. Goodwin. Engineering Projects for Young Scientists. New York: Franklin Watts, 2000.
Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.
Friedhoffer, Robert. Sound. Illustrated by Richard Kaufman and Linda Eisenberg; photographs by Timothy White. New York: F. Watts, 1992.
Gardner, Robert. Science Projects About Sound. Berkeley Heights, NJ: Enslow Publishers, 2000.
Internet Resources for Sound and Light (Web site). <http://electro.sau.edu/SLResources.html> (April 25, 2001).
"Music and Sound Waves" (Web site). <http://www.silcom.com/~aludwig/musicand.htm> (April 28, 2001).
Oxlade, Chris. Light and Sound. Des Plaines, IL: Heinemann Library, 2000.
Physics Tutorial System: Sound Waves Modules (Web site). <http://csgrad.cs.vt.edu/~chin/chin_sound.html> (April 25, 2001).
"Sound Waves and Music." The Physics Classroom (Web site). <http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/soundtoc.html> (April 28, 2001).
"What Are Sound Waves?" (Web site). <http://rustam.uwp.edu/GWWM/sound_waves.html> (April 28, 2001).
| Sci-Tech Encyclopedia: Acoustics |
The science of sound, which in its most general form endeavors to describe and interpret the phenomena associated with motional disturbances from equilibrium of elastic media. An elastic medium is one such that if any part of it is displaced from its original position with respect to the rest, as for example by an impact, it will return to its original state when the disturbing influence is removed. Acoustics was originally limited to the human experience produced by the stimulation of the human ear by sound incident from the surrounding air. Modern acoustics, however, deals with all sorts of sounds which have no relation to the human ear, for example, seismological disturbances and ultrasonics.
Basic acoustics may be divided into three branches, namely, production, transmission, and detection of sound. Any change of stress or pressure producing a local change in density or a local displacement from equilibrium in an elastic medium can serve as a source of sound. Transmission of sound takes place through an elastic medium by means of wave motion. The most important sound waves are harmonic waves, defined as waves for which the propagated disturbance at any point in its path varies sinusoidally with time with a definite frequency or number of complete cycles per second (the unit being the hertz). Acoustics deals with waves of all frequencies, but not all frequencies are audible by human beings, for whom the average range of audibility extends from 20 to 20,000 Hz. Sound below 20 Hz is referred to as infrasonic, and that above 20,000 Hz is called ultrasonic.
The detection of sound is made possible by the incidence of transmitted sound energy on an appropriate acoustic transducer, such as the ear. For modern applied acoustics, transducers such as the microphone, based on the piezoelectric effect, are widely used. Generally speaking, any transducer used as a source of sound is also available as a detector, though the sensitivity varies considerably with the type.
| Music Encyclopedia: Acoustics |
The science of sound and hearing. It treats the sonic qualities of rooms and buildings, and the transmission of sound by the voice, musical instruments or electric means.
Sound is caused by vibration, which is communicated by the sound source to the air as fluctuations in pressure and then to the listener's ear-drum. The faster the vibration (or the greater its ‘frequency’), the higher the pitch. The greater the amplitude of the vibration, the louder the sound. Most musical sounds consist not only of regular vibration at one particular frequency but also vibration at various multiples of that frequency. The frequency of middle C (c′) is 256 cycles per second (or Hertz, abbreviated Hz); but when one hears middle C there are components of the sound vibrating at 512 Hz, 768 Hz etc (see Harmonics). The presence and relative strength of these harmonics determine the quality of a sound. The difference in quality, for example. between a flute, an oboe and a clarinet playing the same note is that the flute's tone is relatively ‘pure’ (i.e. has few and weak harmonics), the oboe is rich in higher harmonics and the clarinet has a preponderance of odd-numbered harmonics. Their different harmonic spectra are caused primarily by the way the sound vibration is actuated (by the blowing of air across an edge with the flute, by the oboe's double reed and the clarinet's single reed) and by the shape of the tube. Where the player's lips are the vibrating agent, as with most brass instruments, the tube can be made to sound not its fundamental note but other harmonics by means of the player's lip pressure.
The vibrating air column is only one of the standard ways of creating musical sound. The longer the column the lower the pitch; the player can raise the pitch by uncovering holes in the tube. With the human voice, air is set in motion by means of the vocal cords, folds in the throat which convert the air stream from the lungs into sound; pitch is controlled by the size and shape of the cavities in the pharynx and mouth. For a string instrument, such as the violin, the guitar or the piano, the string is set in vibration by (respectively) bowing, plucking or striking; the tighter and thinner the string, the faster it will vibrate. By pressing the string against the fingerboard and thus making the operative string-length shorter, the player can raise the pitch. With a percussion instrument, such as the drum or the xylophone, a membrane or a piece of wood is set in vibration by striking; sometimes the vibration is regular and gives a definite pitch but sometimes the pitch is indefinite.
In the recording of sound, the vibration patterns set up by the instrument or instruments to be recorded are encoded by analogue (or, in recent recordings. digitally) in terms of electrical impulse. This information can then be stored, in mechanical or electrical form; it can then be decoded, amplified and conveyed to loudspeakers which transmit the same vibration pattern to the air.
The study of the acoustics of buildings is immensely complicated because of the variety of ways in which sound is conveyed, reflected, diffused, absorbed etc. The design of buildings for performances has to take account of such matters as the smooth and even representation of sound at all pitches in all parts of the building, the balance of clarity and blend and the directions in which reflected sound may impinge upon the audience. The use of particular materials (especially wood and artificial acoustical substances) and the breaking-up of surfaces, to avoid certain types of reflection of sound, play a part in the design of concert halls, which however remains an uncertain art in which experimentation and ‘tuning’ (by shifting surfaces, by adding resonators etc) is often necessary.
The term ‘acoustic’ is sometimes used, of a recording or an instrument, to mean ‘not electric’: an acoustic recording is one made (normally before 1926) before electric methods came into use, and an acoustic guitar is one not electrically amplified.
| Britannica Concise Encyclopedia: acoustics |
For more information on acoustics, visit Britannica.com.
| Architecture: acoustics |
1. The science of sound, including the generation, transmission, and effects of sound waves.
2. The totality of those physical characteristics of an auditorium or room (such as the size and shape of elements on the walls or ceiling which scatter sound, the amount of sound absorption, and noise level within the room) which affect an individual’s perception, and judgment, of the quality of speech and music produced in the room.
| Columbia Encyclopedia: acoustics |
Bibliography
See J. Backus, The Acoustical Foundations of Music (1969); R. B. Lindsay, Acoustics (1973); A. D. Pierce, Acoustics (1981, repr. 1989).
| History 1450-1789: Acoustics |
When he first mentioned the "Acoustique Art" in his Advancement of Learning (1605), Francis Bacon (1561–1626) was drawing a distinction between the physical acoustics he expanded in the Sylva Sylvarum (1627) and the harmonics of the Pythagorean mathematical tradition. The Pythagorean tradition still survived in Bacon's time in the works of such diverse people as Gioseffo Zarlino (1517–1590), René Descartes (1596–1650), and Johannes Kepler (1571–1630). In Bacon's words: "The nature of sounds, in some sort, [hath been with some diligence inquired,] as far as concerneth music. But the nature of sounds in general hath been superficially observed. It is one of the subtilest pieces of nature" (Bacon, p. 390).
Bacon's "Acoustique Art" was therefore concerned with the study of "immusical sounds" and with experiments in the "majoration in sounds" (p. 451), that is, the harnessing of sounds in buildings (architectural acoustics) by their "enclosure" in artificial channels inside the walls or in the environment (hydraulic acoustics). The aim of Baconian acoustics was to catalog, quantify, and shape human space by means of sound. This stemmed from the echometria, an early modern tradition of literature on echo, as studied by the mathematicians Giuseppe Biancani (1566–1624), Marin Mersenne (1588–1648), and Daniello Bartoli (1608–1685), in which the model of optics was applied in acoustics to the behavior of sound. It was in a sense a historical antecedent to Isaac Newton's (1642–1727) analogy between colors and musical tones in Opticks (1704). Athanasius Kircher's (1601–1680) Phonurgia Nova of 1673 was the outcome of this tradition. Attacking British acoustics traditions, Kircher argued that the "origin of the Acoustical Art" (p. 111) lay in his own earlier experiments with sounding tubes at the Collegio Romano in 1649 and sketched the ideology of a Christian baroque science of acoustics designed to dominate the world by exploiting the "boundless powers of sound" (p. 2).
Seventeenth-century empirical observations and mathematical explanations of the simultaneous vibrations of a string at different frequencies were important in the development of modern experimental acoustics. The earliest contribution in this branch of acoustics was made by Mersenne, who derived the mathematical law governing the physics of a vibrating string. Around 1673 Christiaan Huygens (1629–1695) estimated its absolute frequency, and in 1677 John Wallis (1616–1703) published a report of experiments on the overtones of a vibrating string. In 1692 Francis Robartes (1650–1718) followed with similar findings.
These achievements paved the way for the eighteenth-century acoustique of Joseph Sauveur (1653–1716) and for the work of Brook Taylor (1685–1731), Leonhard Euler (1707–1783), Jean Le Rond d'Alembert (1717–1783), Daniel Bernoulli (1700–1782), and Giordani Riccati (1709–1790), who all attempted to determine mathematically the fundamental tone and the overtones of a sonorous body. Modern experimental acoustics sought in nature, as a physical law of the sounding body, the perfect harmony that in the Pythagorean tradition sprang from the mind of the "geometrizing God." Experimental epistemology in acoustics also influenced the studies of the anatomy and physiology of hearing, especially the work of Joseph-Guichard Duverney (1648–1730) and Antonio Maria Valsalva (1666–1723), that in the nineteenth century gave rise to physiological and psychological acoustics.
Bibliography
Bacon, Francis. Sylva sylvarum. In The Works of Francis Bacon. Edited by J. Spedding, R. L. Ellis, and D. D. Heath, vol. 2, pp. 385–436. London, 1858–1859.
Dostrovsky, Sigalia. "Early Vibration Theory: Physics and Music in the Seventeenth Century." Archive for History of Exact Sciences 14 (1974–1975): 169–218.
Gouk, Penelope Mary. "Acoustics in the Early Royal Society, 1660–1680." Notes and Records of the Royal Society of London 36 (1982): 155–175.
Hunt, Frederick Vinton. Origins in Acoustics: The Science of Sound from Antiquity to the Age of Newton. New Haven and London, 1978.
Kircher, Athanasius. Phonurgia Nova. Kempten, 1673.
—PAOLO GOZZA
| Music: Acoustics |
1. The science relating to the creation and dissipation of sound waves. 2. The way in which sound production is affected by the physical properties of the room or chamber in which they are produced.
| Wikipedia: Acoustics |
Acoustics is the interdisciplinary science that deals with the study of sound, ultrasound and infrasound (all mechanical waves in gases, liquids, and solids). A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering. There is often much overlap and interaction between the interests of acousticians and acoustical engineers.
Hearing is one of the most crucial means of survival in the animal world, and speech is one of the most distinctive characteristics of human development and culture. So it is no surprise that the science of acoustics spreads across so many facets of our society - music, medicine, architecture, industrial production, warfare and more. Art, craft, science and technology have provoked one another to advance the whole, as in many other fields of knowledge.
The word "acoustic" is derived from the Greek word ακουστικός (akoustikos), meaning "of or for hearing, ready to hear"[1] and that from ακουστός (akoustos), "heard, audible"[2], which in turn derives from the verb ακούω (akouo), "I hear"[3]. The Latin synonym is "sonic". After acousticians had extended their studies to frequencies above and below the audible range, it became conventional to identify these frequency ranges as "ultrasonic" and "infrasonic" respectively, while letting the word "acoustic" refer to the entire frequency range without limit.
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In Western society it is sometimes believed as an art for thousands of years. Many books/and websites about musical theory written by Western musicologists mention Pythagoras as the first person studying the relation of string lengths to consonance. However from at least 3000 BC the Chinese before had already a scale based on the knotted positions of overtones which indicated the consonant pitches related to the open string, present at their Guqin[4]. Like the Chinese, Pythagoras wanted to know why some intervals seemed more beautiful than others, and he found answers in terms of numerical ratios representing the harmonic overtone series on a string. Aristotle (384-322 BC) understood that sound consisted of contractions and expansions of the air "falling upon and striking the air which is next to it...", a very good expression of the nature of wave motion. In about 20 BC, the Roman architect and engineer Vitruvius wrote a treatise on the acoustical properties of theatres including discussion of interference, echoes, and reverberation - the beginnings of architectural acoustics.[5]
The physical understanding of acoustical processes advanced rapidly during and after the Scientific Revolution. Galileo (1564-1642) and Mersenne (1588-1648) independently discovered the complete laws of vibrating strings (completing what Pythagoras had started 2000 years earlier). Galileo wrote "Waves are produced by the vibrations of a sonorous body, which spread through the air, bringing to the tympanum of the ear a stimulus which the mind interprets as sound", a remarkable statement that points to the beginnings of physiological and psychological acoustics. Experimental measurements of the speed of sound in air were carried out successfully between 1630 and 1680 by a number of investigators, prominently Mersenne. Meanwhile Newton (1642-1727) derived the relationship for wave velocity in solids, a cornerstone of physical acoustics (Principia, 1687).
The eighteenth century saw major advances in acoustics at the hands of the great mathematicians of that era, who applied the new techniques of the calculus to the elaboration of wave propagation theory. In the nineteenth century the giants of acoustics were Helmholtz in Germany, who consolidated the field of physiological acoustics, and Lord Rayleigh in England, who combined the previous knowledge with his own copious contributions to the field in his monumental work "The Theory of Sound". Also in the 19th century, Wheatstone, Ohm, and Henry developed the analog between electricity and acoustics.
The twentieth century saw a burgeoning of technological applications of the large body of scientific knowledge that was by then in place. The first such application was Sabine’s groundbreaking work in architectural acoustics, and many others followed. Underwater acoustics was used for detecting submarines in the first World War. Sound recording and the telephone played important roles in a global transformation of society. Sound measurement and analysis reached new levels of accuracy and sophistication through the use of electronics and computing. The ultrasonic frequency range enabled wholly new kinds of application in medicine and industry. New kinds of transducers (generators and receivers of acoustic energy) were invented and put to use.
The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations.
The steps shown in the above diagram can be found in any acoustical event or process. There are many kinds of cause, both natural and volitional. There are many kinds of transduction process that convert energy from some other form into acoustical energy, producing the acoustic wave. There is one fundamental equation that describes acoustic wave propagation, but the phenomena that emerge from it are varied and often complex. The wave carries energy throughout the propagating medium. Eventually this energy is transduced again into other forms, in ways that again may be natural and/or volitionally contrived. The final effect may be purely physical or it may reach far into the biological or volitional domains. The five basic steps are found equally well whether we are talking about an earthquake, a submarine using sonar to locate its foe, or a band playing in a rock concert.
The central stage in the acoustical process is wave propagation. This falls within the domain of physical acoustics. In fluids, sound propagates primarily as a pressure wave. In solids, mechanical waves can take many forms including longitudinal waves, transverse waves and surface waves.
Acoustics looks first at the pressure levels and frequencies in the sound wave. Transduction processes are also of special importance.
In fluids such as air and water, sound waves propagate as disturbances in the ambient pressure level. While this disturbance is usually small, it is still noticeable to the human ear. The smallest sound that a person can hear, known as the threshold of hearing, is nine orders of magnitude smaller than the ambient pressure. The loudness of these disturbances is called the sound pressure level, and is measured on a logarithmic scale in decibels. Mathematically, sound pressure level is defined
where Pref is the threshold of hearing and P is the change in pressure from the ambient pressure. The following table gives a few examples of sounds and their strengths in decibels and Pascals[6].
| Example of Common Sound | Pressure Amplitude | Decibel Level |
|---|---|---|
| Threshold of Hearing | 20*10-6 Pa | 0 dB |
| Normal talking at 1m | .002 to .02 Pa | 40 to 60 dB |
| Power lawnmower at 1m | 2 Pa | 100 dB |
| Threshold of Pain | 200 Pa | 134 dB |
Physicists and acoustic engineers tend to discuss sound pressure levels in terms of frequencies, partly because this is how our ears interpret sound. What we experience as "higher pitched" or "lower pitched" sounds are pressure vibrations having a higher or lower number of cycles per second. In a common technique of acoustic measurement, acoustic signals are sampled in time, and then presented in more meaningful forms such as octave bands or time frequency plots. Both these popular methods are used to analyze sound and better understand the acoustic phenomenon.
The entire spectrum can be divided into three sections: audio, ultrasonic, and infrasonic. The audio range falls between 20 Hz and 20,000 Hz. This range is important because its frequencies can be detected by the human ear. This range has a number of applications, including speech communication and music. The ultrasonic range refers to the very high frequencies: 20,000 Hz and higher. This range has shorter wavelengths which allows better resolution in imaging technologies. Medical applications such as ultrasonography and elastography rely on the ultrasonic frequency range. On the other end of the spectrum, the lowest frequencies are known as the infrasonic range. These frequencies can be used to study geological phenomenon such as earthquakes.
A transducer is a device for converting one form of energy into another. In an acoustical context, this usually means converting sound energy into electrical energy (or vice versa). For nearly all acoustic applications, some type of acoustic transducer is necessary. Acoustic transducers include loudspeakers, microphones, hydrophones and sonar projectors. These devices convert an electric signal to or from a sound pressure wave. The most widely used transduction principles are electromagnetism (at lower frequencies) and piezoelectricity (at higher frequencies).
A subwoofer, used to generate lower frequency sound in speaker audio systems, is an electromagnetic device. Subwoofers generate waves using a suspended diaphragm which oscillates, sending off pressure waves. Electret microphones are a common type of microphone which employ an effect similar to piezoelectricity. As the sound wave strikes the electret's surface, the surface moves and sends off an electrical signal.
Countless subfields have been created as we have perfected our understanding of the underlying physics of acoustics. The table below shows seventeen major subfields of acoustics established in the PACS classification system. These have been grouped into three domains: physical acoustics, biological acoustics and acoustical engineering.
| Physical acoustics | Biological acoustics | Acoustical engineering |
|---|---|---|
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 Wikisource has original text related to this article:
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Benade, Arthur (1976), Fundamentals of Musical Acoustics, New York, NY, United States: Dover.
Rayleigh, J. W. S. (1894), The Theory of Sound, New York, NY, United States: Dover.
Wilson, Charles E. (2006), Noise Control, Malabar, FL, United States: Krieger Publishing Company.
Stephens, R. W. B. and Bate, A. E. (1966), Acoustics and Vibrational Physics, 2nd Ed, London, UK: Edward Arnold.
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| Translations: Acoustics |
Français (French)
n. - (Phys) acoustique
Ελληνική (Greek)
n. pl. - ακουστική (επιστήμη)
Português (Portuguese)
n. pl. - acústica (f)
Español (Spanish)
n. - acústica
Svenska (Swedish)
n. pl. - akustik, ljudförhållanden
中文（简体）(Chinese (Simplified))
声学, 音质, 音响效果, 音响学
中文（繁體）(Chinese (Traditional))
n. - 聲學, 音質, 音響效果, 音響學
العربيه (Arabic)
(الجمع) علم الصوت, ألسمعانيه : ألخصائص ألتي تحدد قيمه ألمسرح
עברית (Hebrew)
n. - תנאי השמיעה, תורת הקול, אקוסטיקה, תכונות הפצת הקול (של חדר או אולם)
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